ACQUISITIONS AND DIVESTITURES (NARRATIVE) (Details) £ in Millions						3 Months Ended		9 Months Ended
	Apr. 30, 2018 GBP (£) item	Apr. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Nov. 08, 2017 USD ($)	Apr. 03, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Apr. 30, 2018 USD ($) item	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Other liabilities, noncurrent						$ 130,336,000		$ 130,336,000			$ 95,243,000
Contingent Consideration, at fair value						2,751,000		2,751,000			$ 399,000
Revenue of Acquirees since Acquisition Date, Actual						41,300,000		130,000,000
Income (loss) from operations of Acquirees since Acquisition Date, Actual						1,400,000		10,600,000
Acquired intangible amortization of acquirees since acquisition						1,600,000		4,900,000
Business Acquisition, Pro Forma Revenue						364,900,000	$ 371,700,000	1,094,100,000	$ 1,126,200,000
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations, Net of Tax						5,400,000	17,300,000	16,000,000	$ 53,900,000
SCS
Business Acquisition [Line Items]
Description of Acquired Entity	Strategic Communications ServicesOn April 30, 2018, the Company acquired all of the outstanding equity securities of Strategic Communications Services, Ltd ("SCS"). SCS provides services as a system integrator for multichannel contact center platforms, including CISCO. The Company offers in-house, managed and outsourced network, information, communications and contact center services to leading brands throughout Europe. This business will be integrated into the Company's CTS segment.	Strategic Communications ServicesOn April 30, 2018, the Company acquired all of the outstanding equity securities of Strategic Communications Services, Ltd ("SCS"). SCS provides services as a system integrator for multichannel contact center platforms, including CISCO. The Company offers in-house, managed and outsourced network, information, communications and contact center services to leading brands throughout Europe. This business will be integrated into the Company's CTS segment.
Payments to Acquire Businesses	£ 4.4	$ 6,100,000
Total purchase price										$ 9,511,000
Future Value of Liabilities Incurred From Business Acquisitions										2,900,000
Contingent Consideration, at fair value						2,751,000		2,751,000		$ 4,100,000
SCS | Discount Rate
Business Acquisition [Line Items]
Business combination contingent consideration measurement input | item	0.047									0.047
SCS | Other accrued expenses
Business Acquisition [Line Items]
Contingent Consideration, at Fair Value, Current Portion						700,000		700,000
SCS | Other Noncurrent Liabilities [Member]
Business Acquisition [Line Items]
Contingent Consideration, at Fair Value, Current Portion						$ 2,100,000		2,100,000
SCS | United Kingdom, Pounds
Business Acquisition [Line Items]
Contingent Consideration, at Fair Value, Current Portion | £	£ 3.0
BH
Business Acquisition [Line Items]
Date of Acquisition			Mar. 31, 2018
Description of Acquired Entity			Berkshire Hathaway Specialty ConciergeOn March 31, 2018, the Company, through its subsidiary Percepta, acquired certain assets from Berkshire Hathaway Specialty Concierge, LLC ("BH") related to a customer engagement center and the related customer contracts. This acquisition is being accounted for as a business combination. These assets will be integrated into the Company's CMS segment.
Total purchase price			$ 1
BH | Customer Relationships [Member]
Business Acquisition [Line Items]
Fair Value of Assets Acquired			257,000
BH | Lease Agreements [Member]
Business Acquisition [Line Items]
Fair Value of Assets Acquired			330,000
BH | Fixed Assets
Business Acquisition [Line Items]
Fair Value of Assets Acquired			98,000
Motif [Member]
Business Acquisition [Line Items]
Date of Acquisition				Nov. 08, 2017
Percentage of Voting Interests Acquired				70.00%
Description of Acquired Entity				MotifOn November 8, 2017, the Company agreed to acquire all of the outstanding shares in Motif, Inc., a California corporation ("Motif"). Motif is a digital trust and safety services company serving eCommerce marketplaces, online retailers, travel agencies and financial services companies. Motif provides omni-channel community moderation services via voice, email and chat from delivery centers in India and the Philippines via approximately 2,700 employees. Motif will be integrated into the Company's CMS segment.
Payments to Acquire Businesses				$ 46,800,000
Total purchase price				$ 73,438,000
Other liabilities, noncurrent			$ 35,800,000
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life				11 years
Connextions
Business Acquisition [Line Items]
Date of Acquisition					Apr. 03, 2017
Total purchase price					$ 77,712,000
Payments for Previous Acquisition							$ 1,800,000
Proceeds from Previous Acquisition								$ 4,100,000
Cost of Acquired Entity, Up Front Cash Consideration					$ 80,000,000
Connextions | Customer Management Services [Member]
Business Acquisition [Line Items]
Description of Acquired Entity					Connextions, Inc., a health care customer service provider company, from OptumHealth Holdings, LLC. Connextions has been integrated into the health care vertical of the CMS segment of the Company. Connextions employed approximately 2,000 at several centers in the U.S.
Connextions | Customer Relationships [Member]
Business Acquisition [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life					12 years